1. Field of Invention
The present invention relates to an improved laser scanning system wherein first and second derivative signals derived from analog scan data signals generated therewithin are processed in an improved manner so that the effects of thermal noise and substrate/paper noise alike are minimized in diverse laser scanning environments including multiple focal zone scanning systems and large depth of field scanning systems alike.
2. Brief Description of State of the Art
Code symbol scanners are widely used in diverse environments for purposes of object identification, data-entry and the like.
During operation of such machines, a focused light beam is produced from a light source such as a visible laser diode (VLD), and repeatedly scanned across the elements of the code symbol attached, printed or otherwise fixed to the object to be identified. In the case of bar code scanning applications, the elements of the code symbol consists of a series of bar and space elements of varying width. For discrimination purposes, the bars and spaces have different light reflectivity (e.g. the spaces are highly light-reflective while the bars are highly light-absorptive). As the laser beam is scanned across the bar code elements, the bar elements absorb a substantial portion of the laser beam power, whereas the space elements reflect a substantial portion thereof. As a result of this scanning process, the intensity of the laser beam is modulated in accordance with the information structure encoded within the scanned bar code symbol. As the laser beam is scanned across the bar code symbol, a portion of the reflected light beam is collected by optics within the scanner. The collected light signal is subsequently focused upon a photodetector within the scanner which generates an analog electrical output signal which can be decomposed into a number of signal components, namely: a digital scan data signal having first and second signal levels, corresponding to the bars and spaces within the scanned code symbol; ambient light noise produced as a result of ambient light collected by the light collection optics of the system; thermal noise produced as a result of thermal activity within the signal detecting and processing circuitry; and xe2x80x9cpaperxe2x80x9d or substrate noise produced as a result of the microstructure of the substrate in relation to the cross-sectional dimensions of the focused laser scanning beam. The analog scan data signal has positive-going transitions and negative-going transitions which signify transitions between bars and spaces in the scanned bar code symbol. However, as a result of such noise components, the transitions from the first signal level to the second signal level and vice versa are not perfectly sharp, or instantaneous, as in the underlying digital scan data signal. Consequently, it is difficult to determine the exact instant that each binary signal level transition occurs in the detected analog scan data signal.
It is well known that the ability of a scanner to accurately scan a bar code symbol and accurately produce digital scan data signals representative of a scanned bar code symbol in noisy environments depends on the depth of modulation of the laser scanning beam. The depth of modulation of the laser scanning beam, in turn, depends on several important factors, namely: the ratio of the laser beam cross sectional dimensions at the scanning plane to the width of the minimal bar code element in the bar code symbol being scanned, and (ii) the signal to noise ratio (SNR) in the scan data signal processor at the stage where binary level (1-bit) analog to digital (A/D) signal conversion occurs.
As a practical matter, it is not possible in most instances to produce analog scan data signals with precisely-defined signal level transitions. Therefore, the analog scan data signal must be further processed to precisely determine the point at which the signal level transitions occur.
Hitherto, various circuits have been developed for carrying out such scan data signal processing operations. Typically, signal processing circuits capable of performing such operations include filters for removing unwanted noise components, and signal thresholding devices for rejecting signal components which do not exceed a predetermined signal level.
One very popular approach for converting analog scan data signals into digital scan data signals is disclosed in U.S. Pat. No. 4,000,397, incorporated herein by reference in its entirety. In this US Letters Patent, a method and apparatus are disclosed for precisely detecting the time of transitions between the binary levels of encoded analog scan data signals produced from various types of scanning devices. According to this prior art method, the first signal processing step involves double differentiating the analog scan data input signal Sanalog to produce a second derivative signal Sanalog Then the zero-crossings of the second derivative signal are detected, during selected gating periods, to signify the precise time at which each transition between binary signal levels occurs. As taught in this US Patent, the selected gating periods are determined using a first derivative signal Sanalog formed by differentiating the input scan data signal Sanalog. Whenever the first derivative signal Sanalog exceeds a threshold level using peak-detection, the gating period is present and the second derivative signal Sanalog is detected for zero-crossings. At each instant a second-derivative zero-crossing is detected, a binary signal level is produced at the output of the signal processor. The binary output signal level is a logical when the detected signal level falls below the threshold at the gating interval, and a logical xe2x80x9c0xe2x80x9d when the detected signal level falls above the threshold at the gating interval. The output digital signal Sdigital produced by this signal processing technique corresponds to the digital scan data signal component contributing to the underlying structure of the analog scan data input signal Sanalog 
While the above-described signal processing technique describes a simple way of generating a digital scan data signal from a corresponding analog scan data signal, this method has a number of shortcomings and drawbacks.
In particular, thermal as well as xe2x80x9cpaperxe2x80x9d or substrate noise imparted to the analog scan data input signal Sanalog tends to generate zero-crossings in the second-derivative signal Sanalog in much the same manner as does binary signal level transitions encoded in the input analog scan data signal Sanalog.
Consequently, the gating signal mechanism disclosed in US Patent No. 4,000,397 allows xe2x80x9cfalsexe2x80x9d second-derivative zero-crossing signals to be passed onto the second-derivative zero-crossing detector thereof, thereby producing erroneous binary signal levels at the output stage of this prior art signal processor. In turn, error-ridden digital data scan data signals are transmitted to the digital scan data signal processor of the bar code scanner for conversion into digital words representative of the length of the binary signal levels in the digital scan data signal. This can result in significant errors during bar code symbol decoding operations, causing objects to be incorrectly identified and/or erroneous data to be entered into a host system.
Also, when scanning bar code symbols within large scanning fields volumes having multiple focal zones, as taught in co-applicant""s PCT International Patent Publication No. WO 97/22945 published on Jun. 26, 1997, Applicants have observed that the effects of paper/substrate noise are greatly amplified when scanning bar code symbols in the near focal zorie(s) of the system, thereby causing a significant decrease in overall system performance. In the far out focal zones of the scanning system, Applicants have observed that laser beam spot speed is greatest and the analog scan data signals produced therefrom are time-compressed relative to analog scan data signals produced from bar code symbols scanned in focal zones closer to the scanning system. Thus, in such prior art laser scanning systems, Applicants"" have provided, between the first and second differentiator stages of the scan data signal processor thereof, a low-pass filter (LHF) having cutoff frequency which passes (to the second differentiator stage) the spectral components of analog scan data signals produced when scanning bar code elements at the focal zone furthest out from the scanning system. While this technique has allowed prior art scanning systems to scan bar codes in the far focal zones of the system, it has in no way addressed or provided a solution to the problem of increased paper/substrate noise encountered when scanning bar code symbols in the near focal zones of such laser scanning systems.
Thus, there is a great need in the art for an improved laser scanning system wherein first and second derivative signals derived from analog scan data signals generated therewithin are processed so that the effects of thermal and paper noise encountered within the system are significantly mitigated.
Accordingly, it is a primary objective of the present. invention to provide an improved laser scanning system, wherein first and second derivative signals from analog scan data signals produced therewithin are processed so that the effects of thermal and paper noise encountered within the system are significantly mitigated.
Another object of the present invention is to provide such an improved laser scanning system, wherein the beam spot speed of the laser beam varies as a function of distance from the system, or focal zone of the system.
Another object of the present invention is to provide an improved laser scanning system, wherein the scan data signal processor has improved performance throughout the depth of field of the scanning system by automatically tuning the scan data signal processor to an optimum setting for the focal zone being scanned at each moment of scanning system operations.
Another object of the present invention is to provide an improved laser scanning system, wherein a variable first derivative signal pass-band filter is employed having pass-band filter characteristics that are dynamically controlled by the focal distance of the laser scanning beam producing the analog scan data signal being produced.
Another object of the present invention is to provide an improved laser scanning system, wherein a different pass-band filter is dynamically switched into operation for pass-band filtering the first derivative of analog scan data signals produced by laser scanned bar code symbols within each predefined focal zone in the laser scanning system, in order to filter out spectral components of paper noise residing outside the frequency spectrum of the analog scan data signal scanned within the predefined focal zone.
Another object of the present invention is to provide such an improved laser scanning system, which can be a holographic laser scanning systems, a polygonal-type laser scanning system as well as any other type of laser scanning system having multiple focal zones or a large depth-of-field.
Another object of the present invention is to provide an improved laser scanning system, wherein a time-domain non-linear substrate noise filter is employed before the first derivative signal generation stage of the processor so as to produce, as output, a substantially fixed zero-reference signal level whenever a signal level indicative of the substrate is detected, and the signal level of the analog scan data signal whenever a signal level indicative of a bar code element (e.g. dark bar) is detected.
Another object of the present invention is to provide a laser scanning system which employs a signal processor having a bar code element detector for automatically enabling the second-derivative zero-crossing detector employed therein.
Another object of the present invention is to provide an improved signal processing method, wherein detection of second derivative signal zero-crossings is automatically activated (i.e. enabled) upon detection of bar element data encoded within the analog scan data signal, thereby preventing the detection of zero-crossings in the second derivative signal caused by thermal and paper noise during bar code scanning operations.
Another object of the present invention is to provide a novel method of reading bar code symbols, wherein after automatically enabling the detection of zero-crossings in the second derivative signal, second-derivative zero-crossing detection is automatically disabled after a predetermined time period, and automatically re-enabled after redetection of subsequent bar code elements in the same bar code symbol or in subsequently scanned bar code symbols.
Another object of the present invention is to provide an improved method of processing analog scan data signals, wherein gating signals for second derivative zero-crossing detection are automatically generated only when bar code element data is detected in the analog scan data input signal, thereby substantially improving the overall performance of the signal processor in the presence of thermal and paper noise.
Another object of the present invention is to provide a novel laser scanning system capable of being used in diverse types of bar code scanning environments (e.g. where data element stitching is employed).
Another object of the present invention is to provide a novel laser scanning system, wherein the scan data signal processor employs a real-time bar code element detector for enabling a second derivative zero-crossing detector in response to the detection of the presence and absence of bar code element data encoded within the analog scan data input signal.
Another object of the present invention is to provide a multi-focal zone laser scanning system which employs a scan data signal processor that allows reading of bar code symbols having bar code elements substantially narrower than the beam cross-section of the laser scanning beam.
Another object of the present invention is to provide a multi-focal zone laser scanning bar code symbol reading system, in which the ratio of the minimum laser beam cross-section dimension (MBD) to the minimum bar element width (MBW) in each focal zone of the system is greater than or equal to 2.0.
Another object of the present invention is to provide a multi-focal zone laser scanning system, wherein a scan data signal processor is used having a higher overall signal-to-noise ratio (SNR), thereby requiring less modulation of the laser scanning beam during scanning operations, and decreasing the effective laser beam diameter (i.e. beam spot size) at each focal zone in the system, and thus increasing the bar code scanning resolution of the system.
Another object of the present invention is to provide a multi-focal zone scanning system, wherein without increasing or otherwise changing the laser beam power characteristics, the length of each focal zone in the system can be increased to allow either more overlap between adjacent focal zones, or a larger overall depth of field in the system.
Another object of the present invention is to provide a laser scanning system that has an increased depth of field without increasing the power level of the laser scanning beams, or adding additional focal zones to the system.
Another object of the present invention is to provide a laser scanning system which employs a scan data signal processor having a plurality of first derivative signal pass-band filter structures that are electronically-switched into operation in response to control signals derived from information about the focal distance of the laser scanning beam at each instant in time.
Another object of the present invention is to provide a multi-focal zone laser scanning bar code symbol reading system, wherein each scan data producing channel includes a scan data signal processor which employs a variable first derivative signal pass-band filter dynamically controlled by the focal distance of the laser scanning beam producing the analog scan data signal being processed.
Another object of the present invention is to provide a laser scanning bar code symbol reader employing a variable first derivative signal pass-band filter structure having frequency characteristics that are controlled in a real-time manner by measuring the time duration of binary signal levels in digital scan data signals produced in response to laser scanning bar code symbol elements located within the scanning range of the system.
Another object of the present invention is to provide a laser scanning bar code symbol reader employing a variable second derivative signal pass-band filter structure having frequency characteristics that are controlled in a real-time manner by measuring the time duration of binary signal levels in digital scan data signals produced in response to laser scanning bar code symbol elements located within the scanning range of the system.
Another object of the present invention is to provide such a laser scanning bar code symbol reading system, wherein time measurement of bar code symbol elements is carried out using an application specific integrated circuit (ASIC) chip that compares real-time measurement of binary signal levels in digital scan data signals with predetermined time measures thereof, stored in an EPROM or like device, for bar code symbols of different resolutions and scanning distances from the bar code symbol reading system.
Another object of the present invention is to provide such a bar code symbol reading system, wherein a rotating polygon-type mechanism is used to scan the laser scanning beam over the scanning field or volume of the system.
Another object of the present invention is to provide such a bar code symbol reading system, (wherein a variable laser beam focusing mechanism and rotating polygon-type mechanism are used to produce an X-bar or like laser scanning pattern at varying depths of focus over the scanning field of the system.
Another object of the present invention is to provide such a laser scanning bar code symbol reading system mounted over a high-speed conveyor-belt system in order to identify packages, parcels and the like transported therealong in a highly reliable manner.
These and other objects of the present invention will become apparent hereinafter and in the claims to invention.